The invention relates to the reproduction of masters or originals.
Masters or originals in the form of photographs frequently exhibit large variations in brightness. Paper copies or prints of such masters are overexposed in the light areas or underexposed in the dark areas. As a result, details and fine structures are poorly visible, or no longer visible, in the copies.
To improve the quality of a copy made from a master with large variations in brightness, the German Patent Publication No. 40 40 498 proposes to mask the master with an unsharp mask. The unsharp mask reduces the overall contrast while providing greater contrast for the details. This is different from a sharp mask which makes the edges sharper but decreases the contrast for the details. The German Patent Publication teaches a mask with very low resolution, i.e., a mask which produces a very unsharp image of the master.
It has been found that the method of the German Patent Publication yields outstanding results with certain copiers or printers but that these results depend on a number of parameters other than the degree of unsharpness of the mask. For example, highly directional copy light tends to reduce copy quality. Good copy quality can be maintained by increasing the unsharpness of the mask. Alternatively, the reduction in copy quality can be corrected by changing from one objective to another having a different aperture angle. Likewise, the distance between master and copy material, the distance between mask and master, and the gamma of the copy material all affect copy quality.
No quantifiable relationship between these parameters is known.